The present invention relates to a liquid crystal apparatus.
FIG. 4 is a plan view showing a reflection liquid crystal display apparatus 100 associated with the present invention. FIG. 5 is a longitudinal sectional view taken along a line B--B in FIG. 4. This apparatus includes an array substrate 110 on which thin-film transistors (to be referred to as TFTs hereinafter) 114 are arranged as switching elements in the form of a matrix, a counter-substrate 130 on which a transparent counter-electrode 136 is formed, and a liquid crystal layer 150 sandwiched between the substrates 110 and 130.
On the array substrate 110, a plurality of scanning lines 113 and a plurality of signal lines 119 run. The scanning lines 113 run along the column direction. The gates of TFTs 114 of each row are commonly connected to one scanning line 113. The signal lines 119 run in the row direction perpendicular to the column direction. One terminal of the source and drain of each TFT 114 of each row is commonly connected to one signal line 119. The other terminal of each TFT 114 forms a capacitance for storing a signal together with a signal storage capacitance line 116 through an insulating film, and is connected to a reflection pixel electrode 120 (to be described later) through a contact 125.
An organic insulating layer 118 is formed on the upper surfaces of the scanning lines 113, the signal storage capacitance lines 116, and the TFTs 114. The reflection pixel electrodes 120 are formed on the upper surface of the organic insulating layer 118 in correspondence with the respective pixels.
A color filter 134 and the common electrode 136 are stacked on the upper surface of a substrate 132. Red, blue, and green portions are arranged on the color filter 134 in nits of pixels. An optical film 188 such as a retardation plate or polarizing plate is bonded to the opposite surface of the substrate 132 to the liquid crystal layer 150.
As described above, the reflection liquid crystal display apparatus 100 has the reflection pixel electrodes 120 formed on the upper surfaces of the TFTs 114 and signal storage capacitance lines 116 through the organic insulating layer organic insulating layer 118. This can increase the area of the reflection pixel electrodes 120 by which light incident from above in FIG. 4 is reflected. As a consequence, a bright image can be displayed even in a dark place without any backlight.
The following problem, however, arises in the above liquid crystal display apparatus. As shown in FIG. 6A, when a raster window 302 is displayed on a green halftone background 301, vertical crosstalk occurs at an end portion of the window 302. Referring to FIG. 4, since the reflection pixel electrode 120 and the signal line 119 oppose through the organic insulating layer 118, they are coupled via a capacity. However, only one signal line 119 that is connected to a given reflection pixel electrode 120 is coupled via a capacity to this electrode 120.
For this reason, even on the green halftone background 301 to be uniformly display in green, owing to capacitive coupling, the signal lines 119 have different influences on the reflection pixel electrode 120 at a pixel 303 adjacent to a lower portion of the raster window 302 and a pixel 304 spaced apart from the raster window 302. More specifically, at the pixel 304 adjacent to the raster window 302 in the vertical direction, since a signal potential is applied to the signal line 119 to display the raster window 302, the reflection pixel electrode 120 is influenced by the signal line 119 through the capacitive coupling.
Assume that a voltage VP1 is applied to the pixel 303 owing to the influence of the raster window 302, and a prescribed voltage VP0 is applied to the pixel 304, as shown in FIG. 6B. The pixel potentials respectively applied to the pixels 303 and 304 change with time, as shown in FIG. 7.
Let Vsg be the signal potential for a halftone image, Vsb be the signal potential for a black image, and Vsw be the signal potential for raster window display. In addition, let Pco be the coupling factor between a given pixel and a signal line connected thereto through a TFT, and Pci be the coupling factor between the ith signal line adjacent to this signal line and the corresponding pixel. Then, the voltages VP1 and VP0 can be expressed as EQU VP0=Vsg (1) EQU VP1=Vsg+Pc0(Vsb-Vsg)+Pc1(Vsw-Vsg)+Pc2(Vsb-Vsg)+Pc3(Vsw-Vsg)+ . . . =Vsg+(Pc0+Pc2+PC4+ . . . )(Vsb-Vsg)+(Pc1+Pc3+PC5+ . . . )(Vsw-Vsg) (2)
As a result, since the effective voltage at the pixel electrode 120 at the pixel 303 differs from that at the pixel electrode 120 at the pixel 304, and a luminance difference is produced, crosstalk occurs. This problem also arises in a transmission liquid crystal display apparatus.